1. Field of Invention
The invention relates to a multi-channel heat exchanger with high-pressure channels and low-pressure channels and a connection unit for the connection of the multi-channel heat exchanger to other components, for example, an air conditioning unit or heat pump.
2. Related Technology
Below, a heat exchanger is called a multi-channel heat exchanger when the heat-transferring fluids flow through it in two or more in separate channels. The channels are arranged inside of and in the direction of the longitudinal extension or length of the cylindrical multi-channel heat exchanger. The heat exchanger is manufactured preferably by extrusion and used, for example, as an inner heat exchanger with a suitable connection unit within a refrigeration process.
Inner heat exchangers are used, for example, in vehicle air conditioning units, in order to enhance the efficiency of the refrigeration process. The principle of the inner heat exchanger is to transfer heat from the refrigerant with the higher pressure to the refrigerant with the lower pressure. The involved reduction or increase, respectively, of the enthalpy allows the system to achieve a higher refrigeration capacity.
Because of the limited space in a motor vehicle it is desired to also design the components of the vehicle's air conditioning unit, including the inner heat exchanger, such that they need the smallest possible space. That implies also that the surfaces available for heat transfer have to be optimized relative to the size of the inner heat exchanger.
If CO2 is used as the refrigerant, the cross-sections of the refrigerant lines and channels in the heat exchanger must be small because of the high pressure of the refrigerant. Therefore it is necessary, depending on the structural design of the inner heat exchanger and particularly of the radial cross-sections, to provide suitable connection units that can be used to very easily connect, for example, tube lines to the high-pressure channels and to the low-pressure channels.
U.S. Pat. No. 6,098,704 discloses a space optimized coaxial heat exchanger for the heating of the air of a vehicle's interior, where a finned inner tube is inserted in a finned outer tube and whereby the outer wall of the inner tube positively and non-positively adjoins the radial fins of the outer tube. The fluids used for the heat transfer (a water-glycol mixture and a refrigerant) flow in the inner tube and in the interspaces between the fins of the outer tube. Due to this coaxial arrangement of the tubes, a plurality of small radial channels are created that result in good heat transfer. To separate the fluid flows a tube connector, provided over the entire perimeter of the inner tube, is used thus axially limiting the outer tube. Therefore, the refrigerant is able to flow from the outer tube into the tube connector, to be collected and to be further directed orthogonally to the longitudinal extension of the inner heat exchanger. The disadvantage of the heat exchanger is that due to the finned design of the inner and outer tubes, the heat exchanger cannot be operated with the high pressures occurring, for example, in air conditioning units using CO2 as the refrigerant, for reasons of mechanical strength and safety.
From US 2002/0046830 A1, an inner heat exchanger is known that consists of one single tube and that has one inner channel and several outer channels. The outer channels are separated from each other by separating webs or fins, respectively, with the fins arranged radially to the main axis of the tube. The outer channels are connected to further tubes by a special connection element. For that connection, the connection element encloses the outer tube and therefore limits the axial ends of the outer channels. This connection element is connected to the heat exchanger through O-rings or by cutting ferrules guided in grooves. In addition, this connection element is provided with a hollow needle, through which the fluid flow from the axial channel is led out of the heat exchanger. This type of fixation and structural design of the connection element is very expensive.
Known from U.S. Pat. No. 4,368,777 is a cylindrical non-pressurized heat exchanger. The heat exchanger is a countercurrent unit in which water and a heating gas are passed in separate channels. The annular chamber of the outer channel leads the water and is dosed at its axial end by annular termination regions. The water is supplied through flanges that are located orthogonally to the longitudinal extension of the heat exchanger at the outsides of the outer chamber region. The inner tube leads the heating gas and, in its longitudinal extension, is provided with equally spaced radial fins at its inner and outer sides to improve heat transfer. The outer fins are located in the imaginary prolonged gap between two inner fins. In the flow directions of the water and the heating gas, the outer and inner fins, respectively are linearly enlarged in order to prevent the heat exchanger from being overheated or destroyed during operation. Finned heat exchanger tubes enable good heat transfer in some applications, but are not suitable for the inner heat transfer of a refrigeration process because of the high operational pressures and the resulting channel diameters. Additionally, manufacture of tubes finned at the inner and outer sides, with fins that linearly increase or decrease, respectively, in flow direction, appears to be very complicated and expensive.
What is needed by the manufacturers of vehicle air conditioning units, especially in connection with the use of refrigerants under high pressures such as CO2, is to be able to design high-pressure-resistant heat exchangers with a connection element that are suitable for different geometries of the multi-channel heat exchangers and, furthermore, that can easily be connected to each other.